1. Field of the Invention
The present invention relates to series-resonant ballasting circuits for gas discharge lamps, particularly for situations wherein power to the lamp and ballasting means is provided from a high frequency voltage source.
2. Related Patent Applications
The applicant of the instant patent application filed a related patent application entitled "High Frequency Lighting System" on Aug. 25, 1983 (Ser. No. 526,389).
3. Description of Prior Art
Series-resonant ballasting of fluorescent lamps has been described in several prior publications, two examples of which are: U.S. Pat. No. 3,710,177 to Richard Ward, and U.S. Pat. No. 4,370,600 to Zoltan Zansky.
A basic problem associated with series-resonant ballasting relates to the tendency by a series-resonant circuit to develop extremely high voltages whenever the circuit is inadequately loaded. In a series-resonant fluorescent lamp ballast, the main circuit loading would be the fluorescent lamp. However, prior to lamp ignition, the load represented by the lamp is very small; which results in the development of an extremely high voltage across the lamp just prior to ignition. In fact, this initial extremely high voltage will normally be too high for proper lamp starting.
However, a more important problem relates to the situation where the fluorescent lamp is disconnected from the circuit or otherwise ceases to provide adequate circuit loading. In this case, in the absence of circuit protection means, the circuit voltages developed as a result of so-called Q-multiplication can easily reach levels high enough to cause destruction of the circuit or of the power source.
A partial solution to this problem has been provided by Ward in that he has arranged for the lamp cathodes to be connected in series with one of the reactive elements of the series-resonant circuit; which implies that, if the lamp is removed from its socket, the series-resonant circuit is broken, and the resonant effect ceases. However, this partial solution does not provide protection against the very common end-of-life lamp failure mode: where the lamp remains connected in the circuit, but simply fails to ignite.
A more complete but still partial solution to this problem has been provided by Zansky. He describes an inverter-ballast circuit having a means to limit the maximum voltage that can develop across the components of the series-resonant circuit. However, in Zansky's circuit, if the fluorescent lamp is removed, the voltage limiting action is apt to give rise to a significant continuous power loss; which would be due to the very high level of continuously circulating energy within the inverter-ballast circuit caused by the voltage-limiting action. Moreover, the components the in inverter-ballast circuit must be sized such as to be able to handle on a continuous basis this high level of circulating energy; which implies more costly components than otherwise would be necessary.
Also, it is noted that in Zansky's inverter-ballast circuit, the voltage limiting of the ballasting series-resonant circuit is accomplished with the help of the inverter itself and its DC power supply; which in most realistic circumstances implies a need for relatively close proximity between the inverter-part and the series-resonant ballasting part of the inverter-ballast combination.
Thus, Zansky's partial solution does not apply to situations where the inverter is located a substantial distance away from the series-resonant ballasting means, such as in situations where a single central inverter feeds high-frequency power to a number of lighting fixtures located at differenct spaced-apart places--as in a typical commercial suspended ceiling system.